[GlobalISel][AArch64] Combine unmerge(G_EXT v, undef) to unmerge(v).

When having <N x t> d1, unused = unmerge(G_EXT <2*N x t> v1, undef, N),
it is possible to express it just as unused, d1 = unmerge v1.

It is useful for tackling regressions in arm64-vcvt_f.ll, introduced in
https://reviews.llvm.org/D144670.